More recently, an optical fibre called a microstructured fibre, photonic crystal fibre (PCF) or holey fibre has been developed (a special case of such fibres is sometimes referred to as photonic bandgab fibre (PBG)). This PCF comprises a cladding made of a transparent material in which an array of holes is embedded along the length of the fibre [J. C. Knight, et al., Opt. Lett. 21 (1996) p. 1547. Errata: Opt. Lett. 22 (1997) p. 484]. The holes are commonly arranged transversely in a periodic array and are filled with a material which has a lower refractive index than the rest of the cladding. The centre of the fibre commonly comprises a transparent region which breaks the periodicity of the cladding this region often functions as the core of the fibre. However, in principle this region need not be in the centre of the cross section. Commonly the cross section of the fibre comprises a core region comprising a core region material, surrounded by a cladding region comprising holes (optionally filled with air or a gas), solid or liquid micro-structural elements embedded in a cladding background material both regions extending in a longitudinal direction of the optical fibre. Commonly the core will guide 80% or more of the light in the operating wavelengths of the fibre. Typically, both the core and the cladding are made from pure fused silica and the holes are filled with air. In a variation thereof the PCF comprises transversely arranged rods of another material instead of holes. Such fibres are e.g. disclosed in WO 00/37974 which also discloses the PCFs with transversely arranged holes.
The PCF type are generally produced from rod shaped units which are stacked to form a preform, which thereafter is drawn in one or more steps to form the final optical fibre. In 2D Photonic band gap structures in fibre form”, T. A. Birks et al. “Photonic Band Gap Materials”, Kluwer, 1996 is disclosed a method of producing a preform from rods in the form of capillary tubes by stacking the tubes. A method of fabrication of PCFs is also. described in chapter IV, pp. 115-130 of “Photonic crystal fibres”, Kluwer Academic Press, 2003, by Bjarklev, Broeng, and Bjarklev.
WO 03/078338 discloses a method of producing a preform for a microstructured optical fibre wherein a plurality of elongate elements are placed parallel to each other in a vessel where after at least a portion of said vessel is filled with a silica-containing sol, which is dried and sintered.
Microstructured optical fibres are a relatively new technical field where the properties of the waveguide may be designed with a relatively large degree of freedom. Such fibres are commonly made of pure silica comprising a pattern, often made of holes or doped glass, extending in the longitudinal direction of the fibre. The freedom of design makes such fibres interesting for application requiring specific non-linear properties of the fibre. One such application is supercontinuum generation wherein a fibre based source is cable of generating a wide spectral output. Supercontinuum (SC) generation in microstructured fibres has for several years been studied as a source of spatially coherent broadband light (termed white light or super continuum). While new applications of such sources are continuously discovered, several have already been identified, such as various forms of fluorescent microscopy, laser precision spectroscopy, and optical coherence tomography (OCT). High brightness emission in the visible part of the spectrum is especially important for confocal fluorescent microscopy. However, insufficient power in the short wavelength part of the spectrum has so far kept SC-sources from unveiling their full potential within this field. High-power visible SC-generation has been targeted in the experiments presented here.
Most research has so far been based on seeding the non-linear fibre with femtosecond (fs)-lasers but SC-generation using nanosecond- and picosecond (ps)-lasers has also been demonstrated. The latter greatly reduces both cost and complexity of the system, while maintaining a high repetition rate and efficient SC generation. Furthermore, it is generally possible to generate more spectrally uniform SC-spectra in the ps-domain, where also more powerful seed sources are available leading to correspondingly more powerful continua while staying below the damage threshold of the fibre—In all, ps-systems are often particular attractive for real world applications outside optical research laboratories.
The limitation of the average power/spectral density of the supercontinuum source and the width of the supercontinuum is the damage threshold of the nonlinear fibre. The input facet or the first few millimetre of fibre can be destroyed if the peak power or pulse energy is above the bulk glass or glass-air interface damage threshold and the system will have a catastrophic failure. It has been observed by the present inventors that when the peak power or pulse energy is below this threshold the micro-structured nonlinear fibre may still be observed to degrade over time. This degradation is commonly observed as decreasing power in the visible over time. For commercial applications a long life-time of a supercontinuum light source is critical and such a degradation of the fibre is commonly unacceptable.